Thought experiment

An imagined scenario used to examine a theory’s implications, test an intuition, or clarify a concept without physically carrying it out.

Einstein's elevator: A thought experiment in which a person in a closed elevator cannot distinguish gravity from acceleration. Einstein used it to develop the equivalence principle underlying general relativity.
Counterfactual: A conditional claim about what would have happened under circumstances that did not occur. Thought experiments often change one condition to see how a theory's consequences shift.
Plato's Ring of Gyges: A story in Plato's Republic about a ring that makes its wearer invisible. Plato uses the imagined power to test whether people act justly only from fear of punishment.
Physical experiment: A procedure that manipulates or observes physical conditions to gather empirical evidence. Unlike a thought experiment, it produces observations from an actual intervention or measurement.
Schrödinger's cat: A thought experiment describing a cat entangled with a radioactive atom in a quantum superposition. It dramatizes the measurement problem by applying quantum superposition to a familiar macroscopic object.
Idealization: The deliberate simplification of a system by setting aside features judged irrelevant to an inquiry. An imagined scenario can isolate a principle by removing real-world complications.
Archimedes' principle: A law stating that a body immersed in fluid experiences an upward force equal to the fluid's displaced weight. Archimedes' imagined crown test is a famous account of reasoning about a physical problem without a direct measurement.
Computer simulation: A computational representation that calculates how a modeled system behaves under specified conditions. Simulations calculate consequences; thought experiments derive them through imagined reasoning.
Laplace's demon: A hypothetical intellect that knows every particle's state and the laws governing their motion. It frames the determinist ideal that later debates about physical limits challenge.
Intuition pump: A vivid scenario designed to elicit intuitions that bear on a philosophical question. Many thought experiments guide judgment through carefully chosen details.

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